The United States was seeking clarification from China on its deal earlier this year to build two new civilian nuclear reactors for Pakistan, the State Department said on Tuesday.
"We have asked China to clarify the details of its sale of additional nuclear reactors to Pakistan. This appears to extend beyond cooperation that was grandfathered when China was approved for membership in the Nuclear Suppliers Group," State Department spokesman P.J. Crowley told reporters.
"We believe that such cooperation would require a specific exemption approved by consensus of the Nuclear Suppliers Group," Crowley said.
The United States was expected to oppose the China-Pakistan deal next week at a meeting of the Nuclear Suppliers Group.
The 46-nation group controls trade in "dual-use" nuclear fuel, materials and technology to ensure they are applied only to civilian nuclear energy programs and not diverted into clandestine nuclear weapons work.
The Washington Post reported that China has suggested that the sale is grandfathered from before it joined the NSG in 2004, because it was completing work on two earlier reactors for Pakistan at the time.